The main difference i would say is the development and licensing model. Photo prism is forcing ppl who want to commit to sign a CLA to.give away their rights. Also the community is not really active it is mainly one dev that can change the code license on any given time.

Immich does not have such an agreement and has a huge active contributor community around it. Also Immich is backed by Futo which has its pros and cons.

Imho the biggest pain in self hosting is when a foss product turns evil towards its community and start to practice anti consumer/free selfhosters business practices.

Immich is far less likely to turn evil.